The field of the invention is serial communications networks and, particularly, interface circuits for connecting digital processors to serial communications networks.
There are numerous serial communications networks found in businesses and in factories. In the business environment, for example, local area networks sold under the trademark "Ethernet" link personal computers, display terminals, minicomputers, mainframe computers, printers and disk drives together into a system in which data may be transferred rapidly between its components. Similarly in the factory environment, local area networks sold under the trademark "MAP" link together cell controllers with display terminals, minicomputers, mainframe computers, computer aided design systems, computer aided manufacturing systems, computer aided engineering stations and disk drives. The types of serial communications networks have proliferated, and while efforts have been made to standardize communications in certain environments, there currently are hundreds of different networks in use and the number continues to grow.
When designing digital processors for use in a commercial environment, one must select a strategy for connecting that processor to serial communications networks. For example, where the processor has a backplane, such as "VME BUS" or "MULTIBUS", a circuit board for that backplane may be designed for each communications network. A variety of processor circuit boards and communications interface boards can be designed for the backplane and used together in any combination. On the other hand, where the digital processor does not employ a backplane, the interface to the communications network may be constructed as an integral part of the digital processor. In such case, each digital processor must be designed and constructed to operate on a particular communications network, and if many networks are encountered, a processor with integral interface circuit must be offered for each network. In other words, if a manufacturer offers a series of three digital processors of varying power and capability, and those processors must operate on three different communications networks found in their environment, then six circuit board products must be offered if a common backplane architecture is used, or nine different products must be offered if the communications interface is an integral part of the processor.
Where the manufacturer offers a wide variety of digital processor products of varying architectures and numerous communications networks are encountered in the environment in which those processors are used, difficult choices must be made. For example, the Allen-Bradley Company, assignee of the present application, manufactures small, inexpensive programmable controllers such as those disclosed in U.S. Pat. No. 4,504,927, and programming terminals such as that disclosed in U.S. Pat. No. 4,527,250, which do not employ a backplane. They also offer medium sized programmable controllers such as those disclosed in U.S. Pat. No. 4,443,865 that employ one backplane, large programmable controllers such as that disclosed in U.S. Pat. No. 4,442,504 that employ another backplane, numerical controllers such as that disclosed in U.S. Pat. No. 4,228,495 that employs yet other backplanes, and cell controllers such as that disclosed in U.S. Pat. No. 4,831,582 filed on Nov. 7, 1986, which employs still another backplane. Obviously, it is not commercially feasible to provide communications interface modules or circuits for each of these processor products and for each of the communications networks that may be encountered by them in the factory.